This invention relates to a gas chromatography system and methods for increasing the speed, operational flexibility, and accuracy of gas chromatography procedures.
Gas chromatography is a widely employed technique for the separation and analysis of complex mixtures of volatile organic and inorganic compounds. The mixture is separated into its components by eluting them from a column having a sorbent by means of moving gas.
Gas chromatography procedures can be classified into two major divisions; gas-liquid chromatography and gas-solid chromatography. Gas-liquid chromatography is presently the most widely employed type and incorporates a nonvolatile liquid sorbent coated as a thin layer on an inert support structure, generally a capillary tube. The moving gas phase called the carrier gas flows through the chromatrographic column. The analyte partitions or divides itself between the moving gas phase and the sorbent and moves through the column at a rate dependent upon the partition coefficient or solubility of the analyte components. Various types of columns are employed such as tubular glass or stainless steel capillary tubes. In use, the analyte is introduced at the entrance end of the column within the moving carrier gas stream. The components making up the sample become separated along the column and escape from the exit end of the column at intervals and in concentrations characteristic of the properties of the analyte components. A detector, for example, a thermal conductivity detector or a flame ionization detector (FID) at the exit end of the column responds to the presence of analyte components. Upon combustion of the eluted material in the FID, charged species are formed in the flame. The flame behavior is monitored through a biased ion detector which, along with associated electronics, produces a time versus magnitude trace of the detector output. The trace for a complex mixture includes numerous peaks of varying intensity. Since individual constituents of the analyte produce peaks at characteristic times and whose magnitude is a function of their concentration, much information is gained through an evaluation of the chromatogram.
Gas chromatography systems of the type described above are in wide-spread use today. Although present systems provide excellent performance and utility, this invention seeks to optimize and further enhance the usefulness of the procedure.
Present gas chromatography apparatus and techniques often require significant analysis times necessary to complete the analysis of a single sample. Such time requirements are imposed due to several factors. The use of long columns (i.e. greater than 10 m) increases the time necessary for materials of interest to traverse the entire column. Long column lengths are traditionally necessary since the sample cannot be delivered at the entrance end of the column as a highly concentrated plug of material but rather is introduced over a significant time period. In order to provide acceptable definition in the separated mixture when the sample cannot be introduced as a dense plug, it is necessary to allow the analyte to travel significant distances along the separation column to avoid smearing of the output caused by differences in the time that portions of the analyte are introduced into the column. In addition, for some analytes the process of complete separation is significantly increased due to the presence of relativity high boiling point components which travel very slowly along the separation column. Although all of the significant information desired from the procedure might be obtained within a relatively short time period as the peaks of interest are generated in the chromatogram, it is necessary to wait for these higher boiling point components to be eluted from the column until the next sample can be introduced.
A further time constraint in traditional gas chromatography procedures is the backflushing process. Following separation, it is ordinarily necessary to backflush the column by providing a fluid stream which travels in a direction opposite that which the analyte moves during separation. This process cleanses the column of any analyte components which might remain and enables the column to be used a greater number of cycles. Since the time for backflushing necessary to purge the column is a function of the square of its length, long column gas chromatography systems require a significant time in which to conduct a gas chromatography experiment.
The above mentioned time constraint on conducting gas chromatography evaluations using traditional systems can limit its utility in applications such as process control procedures or gathering large samples of data in order to provide statistical process quality assurance information.
The gas chromatography system and procedures in accordance with the present invention achieve significant reductions in the time necessary to conduct a gas chromatography evaluation. These improvements in evaluation time are attributable to a number of factors. First, the gas chromatograph in accordance with the present invention utilizes a column of relatively short length i.e. less than 10 meters and preferably about 2 meters. In addition, a cryofocussing chamber is used which causes the sample to be trapped in a solid phase and is then vaporized using a novel rapid heating circuit, enabling the sample to be injected into the inlet end of the column as a high density narrow plug. This concentrated sample plug provides acceptable resolution while using a short column.
This invention further reduces analysis time through incorporating a backflush system in which a vacuum pump is used to reverse the flow direction of fluid through the column after the low boiling point components (relative to other components) of the mixture which are of interest have eluted from the column. Accordingly, high boiling point materials which have not traveled a long distance along the column can be vented through backflushing in a comparatively short period of time. This backflushing operation is achieved using a vacuum pump which draws the fluid through the separation column in a reverse direction without requiring the use of compressed gas sources to drive the backflushing flow, which are necessary when using long columns.
Another area for optimization of gas chromatography evaluation provided by this invention relates to the fact that some samples produce chromatograms having a high intensity broadened peak having a long "tail" generally caused by a solvent in the mixture, and hence is referred to as a "solvent peak". Impurity substances which may elute along the solvent peak or along the solvent peak tail may become completely obscured due to their relatively short time duration and small magnitude as compared to that of the solvent peak. Accordingly, significant data can be obscured.
Systems are presently known for avoiding the problem of data obscuration caused by the presence of solvents mentioned above. For example, so-called heart cutting procedures using multiple columns have been used. However, these procedures significantly complicate the gas chromatography equipment needed, require more control inputs, and significantly increase the time necessary to complete the evaluation.
In accordance with the present invention, a gas chromatography system is provided with means for controlling flow direction through the system element which allows the flow direction through the column to be reversed just at the point where the high concentration solvent components have eluted, but before components of interest following the solvent peak have eluted from the column. These components of interest are returned through the column in a reverse direction and into the cryofocussing device where they are refocused by cooling and subsequently rapidly heated and reinjected into the column. Since these materials are free from at least some of the high concentration material causing the solvent peak the presence of other components of interest are more readily detected upon a second separation process. This process of flow reversal and sample recollection (referred to later as "backflush and retrap" mode) in order to eliminate significant amounts of solvent can be repeated numerous times to provide near complete removal of the solvent materials or materials creating high magnitude, long duration peaks.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.